Use containers to avoid cleanups

Use containers to avoid cleanups

This patch introduces the use of various containers -- std::vector,
std::string, or gdb::byte_vector -- in several spots in gdb that were
using xmalloc and a cleanup.

ChangeLog
2017-08-03  Tom Tromey  <tom@tromey.com>

	* valops.c (search_struct_method): Use gdb::byte_vector.
	* valarith.c (value_concat): Use std::vector.
	* target.c (memory_xfer_partial): Use gdb::byte_vector.
	(simple_search_memory): Likewise.
	* printcmd.c (find_string_backward): Use gdb::byte_vector.
	* mi/mi-main.c (mi_cmd_data_write_memory): Use gdb::byte_vector.
	* gcore.c (gcore_copy_callback): Use gdb::byte_vector.
	* elfread.c (elf_rel_plt_read): Use std::string.
	* cp-valprint.c (cp_print_value): Use gdb::byte_vector.
	* cli/cli-dump.c (restore_section_callback): Use
	gdb::byte_vector.